The use of aqueous-based inks for ink-jet printers is well-known. Such compositions are relatively inexpensive and easy to prepare; typically, the ink comprises water and a glycol or glycol ether, usually diethylene glycol (commonly referred to as the vehicle), and a dye. Typically, the water and glycol ether are present in generally the same proportion and the dye, for example, Food Black 2, is present up to about 6% of the total composition, depending on the desired density of the print.
The prior art inks generally use existing commercial dye salts (cation plus dye anion) as formed, which are simply dissolved in the vehicle and filtered to prepare the ink. Such dyes, which generally contain a plurality of sulfonate or carboxylate anion groups, are designed to form solids in paper or cloth, employing cations such as sodium cations. Consequently, the dyes do not easily remain liquid in the orifice of an ink-jet printer.
Thus, a persistent problem associated with aqueous-based inks is their propensity to crust over a period of time, eventually leading to plugging of the orifice in the printer mechanism from which droplets of ink are expelled in the printing operation. Crusting is the crystallization of the ink around the orifice in the print head, causing partial or full blockage of the orifice, leading to misdirection of the drop (partial blockage) or prevention of drop ejection (full blockage). The crusting problem arises from the evaporation of the water from the ink solvent (vehicle) and the consequent precipitation of the dye salt which has become substantially insoluble as a result of this water loss.
Attempts have been made to solve the crusting problem. Hygroscopic agents have been added to reduce the rate of water evaporation by their ability to pick up water vapor from the air. Exemplary of such hygroscopic agents are water-soluble polymers, alkanol amines, amides and polyhydric alcohols.
While some improvement has been realized with these hygroscopic agents, a total solution to the crusting problems has not yet been achieved. Further, apparently no methods are known to prevent crusting of the anionic dyes in mildly acidic to mildly basic aqueous-based inks (pH 4 to 9). Such crusting is particularly exacerbated by the trend to fabricate printheads with increasingly smaller orifices, on the order of 45 to 60 .mu.m in diameter.
Attempts are continuing to develop inks in which the dye solubility is increased.
Other approaches include developing new dyes for inkjet inks. For example, U.S. Pat. No. 4,557,761 discloses a variety of sulfonate-containing dyes with cations such as sodium, potassium, lithium, ammonium and amine salt cations.
However, as shown above, dyes with sodium cations have certain deficiencies. Dyes with potassium cations also suffer from certain deficiencies, notably evidencing crusting. Dyes with ammonium cations, on the other hand, are not stable. Thus, the cations listed in U.S. Pat. No. 4,557,761 cannot be considered to be equivalent.
Finally, many cations are not suitable for dyes employed in inks used in thermal ink-jet printing. In this instance, a problem known as kogation may occur. Kogation is a coined term, unique to thermal ink-jet printing, and describes the extent of decomposition of the ink on the resistors of thermal ink-jet printers as a consequence of heating. Such heating is used to form droplets of ink, which are propelled toward the substrate.
While sodium-containing dyes evidence crusting problems, they also evidence superior kogation properties. Dyes containing other cations which evidence improved crusting also often evidence poor kogation.
It is known to partially replace sodium cations with lithium cations; see, e.g., Japanese Laid-Open 61-73772 (Applicant: Mitsubishi Kasei kagaku) and Japanese Laid-Open 57-202358 (Applicant: Fujitsu). However, such partial replacement has been found inadequate in dealing with the crusting problem, especially for the smaller orifice sizes on the order of 45 .mu.m. Further, Japanese 57-202358 extols the use of a vehicle having about 25 to 35% water, the balance glycol. Inks employing such vehicles, however, are not suitable for printing on plain paper.